Are you saying that the yellow foliage is on the plants from the moment they sprout and yet still they bloom or that the foliage on the bottom begins to yellow as the plant grows?
Can you post a pic?
Yellow foliage from the get go is likely a bulb problem....likely rot of some sort...but not likely to bloom and would die.
If your plants have other things growing around them and get a certain amount of shade, the foliage on the bottom will yellow as well....I have this on some of my lilies from being planted between the other perennials...it is no big deal and they flower away.
flora